1. Field of the Invention
This invention relates to electrochromic devices which are useful for transmission-type device such as smart windows, reflective-type devices such as antiglare mirrors for automobiles and decorative mirrors and displays.
2. Description of the Prior Art
Japanese Laid-Open Patent Publication No. 63-18336 discloses a method of producing a chromogenic layer used for an electrochromic device by vacuum-depositing or sputtering an inorganic oxide such as tungsten oxide (WO.sub.3) over a transparent electrically conductive film. However, this method leads to elevated production costs due to the involvement of technique under vacuum conditions.
It has been proposed to produce a chromogenic layer by using an electrochromic compound such as viologen and an electron donating compounds in combination without employing a technique involving vacuum conditions.
For example, Japanese Laid-Open Patent Publication No. 9-120088 discloses a solution for an electrochromic device which is obtained by dissolving an electrochromic compound such as viologen and an electron donating compound in a suitable solvent. However, this attempt has a problem that the resulting electrochromic device is deteriorated in electrochromic performances since viologen which is freely present in the solvent, is easily dimerized, leading to precipitation. Furthermore, there is a risk that the liquid medium possibly scatters upon breakage of the device.
Japanese Laid-Open Patent Publication No. 7-70218 discloses a solid chromogenic layer produced by dispersing an electrochromic compound such as viologen and an electron donating compound in a polymer with the aid of a monomer which can be polymerized by irradiating electromagnetic radiation. Since the solid medium is used in this chromogenic layer, there is no risk that the medium scatters upon breakage of the device. However, the viologen is still freely present even in the solid medium and thus easily precipitates by being dimerized, resulting in a failure of completely avoiding the resulting electrochromic device from being deteriorated.
Furthermore, the viologen and electron donating compound are changed in properties by ultraviolet ray penetrating through the substrate of the device, leading to the shortened lifetime thereof.